cpufreq: CPPC: Avoid overflow when calculating desired_perf

This patch fixes overflow issue when calculating the desired_perf.

Fixes: ad38677d (cpufreq: CPPC: Force reporting values in KHz to fix user space interface)
Signed-off-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>